This disclosure is directed to a magnesium calcium oxide composite which is adapted for use, e.g., in desulfurizing steel manufacturing processes. In addition, nodules in molten ferrous metal are altered in shape to improve workability of metal products. Generally, it is undesirable to incorporate sulfur in steel. This material provides an additive which is injected into the steel manufacturing process to remove sulfur. Injectables are added during steel manufacturing through injection lances. Some mangnesium based injectable materials are salt coated. This coating leads to problems with injection line plugging because of hygroscopic nature of the salt coating material. As the injectable material is introduced into the steel, there is a possibility of violent reaction. For instance, the addition of magnesium in particulate form runs the risk of violent reaction. The violent reaction may take the form of bubbling, splattering, or the like. Moreover, finely ground particulate dust is difficult to meter in blast furnace injection processes. A related factor is that finely ground dust injectables create a hazard in handling. If they are finely ground, exposed to high temperatures and have some supply of oxygen available, there is the possibility of explosion. The material (with low carbon or with high carbon), normally molten at temperatures between 1200.degree. and 1800.degree. C.
Another important problem relates to reduction of nodule size. In a molten ferrous metal, graphite forms slivers which may degrade physical characteristics during metal working. The injectable of this disclosure reduces nodule size by changing nodule shape, reducing nodule surface size and forming nodules of spherical shape. Thus, one feature of the injectable is that it operates to nodulariaze the molten ferrous metal.
The injectable of this disclosure is one that can be added to a steel manufacturing process with reduced risk of explosion, reduced dust problems, reduced segregation of Mg and lime, and yet obtain a high degree of sulfur removal. One reference of interest is U.S. Pat. No. 4,139,369. This is a powder mixture of magnesium and selected calcium compounds. No particular method of preparation is set forth. A similar reference is U.S. Pat. No. 4,182,626. This disclosure mentions a method of manufacture in multiple stages to reduce ignition of the magnesium in powder form. Perhaps a more remote reference is U.S. Pat. No. 4,209,325 which is directed to a similar mixture of magnesium, sintered lime and a fluxing agent. Preferred fluxing agents are alumina or selected fluorides. A mixture of magnesium and calcium oxide is set forth in U.S. Pat. No. 4,137,072. This disclosure appears to be directed solely to a mixture and does not reference magnesium-calcium alloys in conjunction with the mixture of magnesium and calcium oxide. Mixtures of Mg powder and lime powder are likely to segreate into Mg-rich and limerich portions during handling and shipping, in contrast to the compositionally-stable mixtures made in accordance with the present invention.
In contrast with these references, this disclosure describes both the product and the method of manufacture thereof. Briefly, the product is a composite of magnesium and calcium oxide, sometimes called lime. As the references above mentioned note, magnesium and various calcium compounds are known steel desulfurizing agents. As the references above also mention, magnesium is rather unstable in finely ground form and various calcium compounds in powder form are relatively inefficient in use or cost. In contrast, the composite of the present disclosure is both a mixture and an alloy. The composite is therefore somewhat brittle and easily ground to powder form without the dust problems of the prior art. Even when in powdered form, the particles are harder to ignite and therefore more easily stored and handled. At the time of injection, there is less violent reaction in the steel. Moreover, the composite of this invention is a highly desirable injection agent substantially free of the problems of hygroscopic water adsorption, potential dust explosions, and the like. Moreover, it seems to be more desirable in that there is an alloy of magnesium and calcium which lends itself readily to steel desulfurization. By contrast, pure magnesium is difficult to grind while this product is easily broken to pieces and processed to size.
Briefly, the process contemplates utilizing magnesium in a molten state, stirring vigorously while introducing lime into the melt, all accomplished under an inert gas layer to thereby form the composite mixture. On cooling, it can be broken and then ground yielding both a mixture of magnesium with calcium oxide and also magnesium and calcium in an alloy.